This invention relates to drum dryers and/or mixers and more particularly to such drum dryers and/or mixers of the type used for the continuous drying and/or mixing of materials such as those used in the road paving arts.
In recent years in the road and highway construction field, substantial attention has been directed toward the use of rotating drum mixers for the production of paving compositions on a continuous basis. Typically, such mixers comprise a rotating drums having internal flights for lifting and tumbling materials within the drum and a source of high volume heated gases aligned with the drum for providing the interior of same with a continuous flow of a hot gaseous medium. A typical process for the use of such a drum mixer in the production of an asphalt paving composition is disclosed in my U.S. Pat. No. 3,832,201 issued Aug. 27, 1974 entitled "Process for Making Asphalt Paving Compositions."
The popularity of drum mixers in recent years has been particularly attributable to the increased concerns for the reduction of the emission of pollutants and irritants from pavement-producing equipment. Federal, state and local regulations and statutes typically require that current producers of asphalt paving compositions comply with strict controls on the amounts of particulates, smoke and the like that are emitted from their operating plants. The process disclosed in my U.S. Pat. No. 3,832,201 minimizes the production of particulates from a drum mixer by the addition of liquified asphalt to unconditioned aggregate close to the inlet end of the mixer in order to capture and coat the fines present at that point.
It has been found in the operation of my process that when it is used with certain types of liquified asphalt, substantial amounts of blue smoke are produced at the output stack. The production of such smoke is associated with the use of those asphalts having relatively low smoking points and is caused by the tumbling of the asphalt through, and consequently subjecting the asphalt to, an extreme high temperature zone typically found along the central axis of an operating drum mixer. The extreme high temperature zone at the central axis is normally present only near the inlet end of the mixer and eventually dissipates at increasing distances into the mixer from the inlet end. Accordingly, when using low smoking point asphalts, the introduction of the asphalt must oftentimes be made at substantial distances downstream from the inlet end of the mixer. Typically, such distances may be on the order of one-third or more of the overall length of the mixer. When the asphalt is added at such downstream positions, certain of the advantages disclosed in my referenced patent are minimized and in fact particulate emission is increased. The latter result follows from the fact that maximum particulate emission suppression is achieved by introducing the asphalt into the mixer as close as possible to the inlet end thereof.
The conservation of paving material has also been of great interest to the paving industry in recent years. With increased costs of aggregates and asphalts, coupled with lessening supplies of both materials, a need exists for new methods and equipment designed to use less of such materials. One suggested solution to the material cost and supply problem is the use of old materials removed from existing roadways as an ingredient to a paving mixture being prepared for a repaving of such existing or other roadway surfaces. The old removed materials are crushed and reduced in size to workable dimensions. Heretofore, the repaving of such an existing roadway involved the removal of the existing roadway paved surface and the replacing of same with a new paved surface produced from new aggregate and asphalt materials. Typically, the older pavement which is removed is discarded as waste material. It has been proposed to recycle such discarded waste material into new paving material, thereby effecting substantial cost savings as well as conservation goals.
Unfortunately, when recycled paving materials are introduced into present pavement mixing plants, the production of excessive amounts of pollutants has ordinarily resulted. In particular, the introduction of recycled pavement, in lieu of aggregate, into a drum mixing process such as that disclosed in my referenced patent, results in the production of excessive amounts of smoke and other irritants. Such pollutant production is once again attributable to the presence of an extreme high temperature zone along the central axis of an operating drum mixer. In the case of the smoking problem attributable to the use of low smoking point asphalts, the introduction of the asphalt substantially downstream in the drum mixer can alleviate the problem. In the case of recycled pavement, however, the old asphalt material is necessarily present immediately at the inlet end of the mixer since it is bonded to the aggregate and cannot economically be removed therefrom.
It is apparent therefore that it is desirable to minimize the extreme high temperature zone present near the central axis of a drum mixer. More particularly, it is desirable to modify the radial temperature distribution present in a drum mixer by minimizing the extreme high temperature zone present near the central axis of such mixer.
In addition to the drum mixer problems noted above, similar problems caused by the extreme high temperature zone along the central axis of a drum are also present when such drums are used solely for drying purposes. Drum dryers have utility in various industries and prior to the development of drum mixers were used in the asphalt industry to dry aggregate materials prior to their introduction into a pug mill type mixer. Many such drum dryer and pug mill mixer combinations exist throughout the world today and each could have similar problems from smoking, etc., if used with recycled materials as discussed above.
An object of this invention, therefore, is to provide an improved drum dryer and/or mixer.
Another object of this invention is to provide an improved drum dryer and/or mixer having lower noxious emissions than those of the prior art.
A more particular object of this invention is to provide an improved drum dryer and/or mixer having a reduced high temperature zone along its central axis and consequently a more uniform radial temperature distribution within its interior.